Reading information from magnetic media may be enhanced by using magneto resistive (MR) head technology. MR read head technology may allow the areal density of information recorded on magnetic media to be greatly increased. Typical track density in a small form factor disk drive may be around two thousand or more tracks per inch.
Because of increasing track density requirements resulting in greater Tracks Per Inch (TPI), Track Misregistration (TMR) budget must be accordingly smaller for high density drives. A TMR budget may be expressed as a tolerance for allowing an MR head position or registration to deviate from a track center before interfering with data recovery. Maximum error-free data recovery may be accomplished by maintaining MR read head registration at track center as consistently as possible.
Because TMR budgets may be tight due to high track densities, offset generated by repetitive and non-repetitive runout (RRO and NRRO) may be particularly troublesome. In addition, deviations in write track and read track centers from a common actuator arm center may increase problems with TMR management. RRO in the form of eccentricities in the circularity of a track being read and NRRO in the form of wobble from bearing irregularities may cause head misregistration which may exceed TMR budget. Unless compensated for in a manner avoiding excessive overshoot or overcorrection which may, in itself, cause misregistration, RRO and NRRO may prevent error-free data recovery without using high tolerance, high cost spindle motors.
RRO may be due to systematic distortion in disk construction, minute warpage of disk surfaces, slightly off-center placement of a disk spindle with respect to a platter, or rotational harmonics. NRRO may be due, as mentioned, to random perturbations and nutations caused by bearing irregularities and the like. Other sources of RRO and NRRO may be related to high rotational speeds of a typical drive.
Typical disk drive speeds are fixed at 5400 RPM and 7200 RPM. Rotational harmonics associated with RRO may therefore be correlated to multiples of fundamental frequency. NRRO on the other hand, may be more difficult to compensate for based on their transient nature. High precision spindle motors may be available with tighter RRO tolerances at a high cost.
Correcting head position may be one method to deal with disk irregularities. While many methods may exist to position a head and compensate for errors in head position, two primary methods may be favored by designers: dedicated and embedded servo methods. Dedicated servo technology uses one side of a disk platter containing positioning information. A single dedicated head may be used to access a servo positioning platter side with other heads being slaved to the dedicated head.
Dedicated servo approaches are wasteful of disk surface especially in systems with fewer platters. Embedded servo technology overcomes limitations associated with dedicated servo by including positioning information on a data track. Servo burst patterns may be used to correct positioning as a head attempts to follow a data track.
As a read head deviates from track center, a pattern may be read which indicates the direction and relative magnitude of displacement from track center. Information from reading a servo burst pattern may be fed back to head positioning electronics and head position may be corrected accordingly.
FIG. 3 is a timing diagram illustrating servo burst pattern, and other servo sector information of the prior art. During manufacturing, servo burst pattern 307 including A, B, C, and D bursts may be written upon a disk surface by a servo writer. Servo burst patterns may be written in servo sector information areas which may appear at regular intervals along concentric tracks from outer tracks to inner tracks.
Disk sectors may appear as pie slice shaped areas of a disk surface comprising increasingly smaller portions of tracks closer to a disk spindle. In addition to servo burst pattern 307, AGC field 303, servo synch field 304, servo gray code field 305, and ID field 306 may be written. Following servo burst pattern 307 is data field 308. An error correcting code 309 (ECC) may be written at the end of servo sector information. Servo burst patterns may be used to generate a position error signal for positioning an actuator arm.
FIG. 4 is a timing diagram illustrating the timing relationship between servo sector information, servo gate signal and burst window of the prior art. Track data 400 may include servo sector information fields. Servo sector information may include a servo burst pattern comprising A, B, C, and D bursts. A, B, C, and D bursts may be written following a servo preamble, a servo address mark, a servo gray code on a single track.
A servo gate signal 401 may be used to control when a servo burst pattern is written by a servo writer. A servo burst window timing signal 402 may control the write timing of A, B, C, and D bursts by a servo writer. The servo writer may perform writing of burst patterns in servo sectors during disk manufacturing. In prior art approaches, global error stored in servo burst data of an outer track may be used to characterize errors related to harmonics for an entire disk.
Problems with servo bursts written by a servo writer may occur if new eccentricities develop or change between drive assembly and drive certification. Moreover, errors may be present in the servo burst information for a particular sector which may cause head tracking to be lost within that particular sector.
It would be desirable for a servo controller capable of calculating an average RRO error value for all tracks. It would also be desirable for a servo controller capable of writing a multitude of information regarding head positioning with respect to track location in a servo parameter field including average RRO for subsequent sectors. Separate servo parameter information for read and write heads written in position corrected fields would be especially desirable. It would be further desirable for a servo controller capable of comparing present servo parameter position error information and making accurate corrections in head position for subsequent sectors to maximize read signal amplitude for both read and write tracks.